The present invention relates to systems including a fuel cell, and more particularly to a system and method for controlling steam in a fuel cell system with a fuel processor.
Fuel processors in fuel cell systems convert hydrocarbon fuel, such as gasoline or methanol, into a rich hydrogen stream. The hydrocarbon fuel is reformed to the hydrogen stream through a series of reactions with steam and/or air. Using exothermic heat, steam is generated, superheated, and fed to the fuel processor. Operation of the fuel processor requires effective balancing of the reaction chemistry. Balancing is accomplished by controlling reactor temperature and pressures, stream composition, and steam generation. The temperature of the heat exchanger affects steam generation.
The fuel processors typically include autothermal reactors, steam reformers, preferential oxidizers (PrOx), and/or water gas shift reactors. The steam that is required by the fuel processor is typically generated in vaporizers, such as a heat exchanger/vaporizer or a combustor/vaporizer. After shutdown, steam that remains in the autothermal reactors, steam reformers, PrOx and/or water gas shift reactors condenses to liquid water. Liquid water that remains in these fuel processor components after shutdown causes problems with catalysts, especially shift catalysts. Liquid water also promotes corrosion in these components. The remaining water adversely impacts durability and causes restart problems at and below freezing temperatures.
A control system and method according to the present invention controls steam in a fuel cell system including a fuel processor. The fuel cell has run, standby and shutdown operating modes. The fuel processor provides reformate to the fuel cell. A pressure sensor generates a pressure signal based on a pressure of steam supplied to the fuel processor. A valve selectively vents steam away from the fuel processor. A controller communicates with the pressure sensor, the fuel cell and the valve and controls the valve based on the operating mode of the fuel cell and the pressure signal.
In other features, the controller opens the valve to prevent steam from reaching the fuel processor during the shutdown mode. The controller closes the valve during the run operating mode. The controller initially closes the valve during the standby mode to maintain steam in vaporizers of the fuel processor for fast startup. During the standby mode, the controller opens the valve if the pressure signal exceeds a first predetermined pressure value. The controller subsequently closes the valve if the pressure falls below a second predetermined pressure value and/or after a first predetermined period elapses.
In still other features, the fuel processor includes at least one of a water gas shift reactor, a steam reformer, PrOx, and an autothermal reactor. A conduit supplies the steam to the fuel processor. A pressure regulator is coupled to the conduit.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.